1. Field of the Invention
This invention relates to a method of obtaining a number of copies transferred from one electrostatic latent image produced on an electric charge carrying member coated on an electrically conductive transfer body.
2. Description of the Prior Art
In prior art electrographic techniques, as means for transferring a picture image produced on a transfer body and developed by toner particles to a transfer receiving medium such as plain paper, a corona discharge transfer system, bias roll transfer system, etc. have been proposed.
In the conventional corona discharge transfer system, the rear surface of the transfer receiving medium is charged with a corona source of ions having a polarity which is opposite to that of toner particles so as to transfer the toned picture image to the plain paper. As a result, if the corona charging voltage is not high, it is difficult to uniformly charge the transfer receiving medium with the corona source of ions and hence it is difficult to eliminate undesirous transfer of the toned image to the transfer receiving medium. In addition, if use is made of a transfer receiving medium formed of plain paper, the plain paper has a resistance value of 10.sup.10 to 10.sup.14 .OMEGA.cm, so that if the corona charging voltage is made too high, a flow of corona ions is introduced through the plain paper into the transfer body, thereby deteriorating the electrostatic latent image produced on the transfer body.
In the conventional bias roll transfer system, an electrically conductive roller with or without an insulating film coated thereon is urged against the rear surface of a transfer receiving medium and a bias voltage is applied between the roller and the transfer receiving medium. In such transfer system, the force for urging the roller against the transfer receiving medium, the amount of toner particles adhered to the transfer body and the fluidity of toner particles cause a center portion of the picture image not to be transferred to the transfer receiving medium, thereby degrading the transfer efficiency. In order to improve the transfer efficiency, if the bias voltage is made high, the electrostatic latent image produced on the transfer body becomes deteriorated in the same manner as in the case of the corona discharge transfer system. As a result, it is difficult, in general, to apply the bias roll transfer system to a high speed transfer process.
The above described conventional transfer systems have been encountered with difficult problems that the electrostatic latent image becomes damaged due to the corona charging voltage and bias voltage and that the toned picture image is considerably disturbed due to discharge which is produced when the transfer receiving medium is detached from the transfer body and hence the quality of the picture becomes degraded or the picture image is overdeveloped to show spots therein. As a result, it is difficult to obtain a number of copies from the sole electrostatic latent image produced on the transfer body.